The present invention relates generally to an imaging optical system and an imaging system that incorporates the same, and more specifically to a small-format phototaking optical system that relies on a solid-state image pickup device such as a CCD or CMOS. For instance, the invention is concerned with a phototaking optical system that can be utilized for digital still cameras, digital video cameras, miniature cameras mounted on cellular phones and personal computers, and surveillance cameras or the like.
In recent years, electronic cameras using solid-state image pickup devices such as CCDs or CMOSs to photo-object subject images have come into wide use in place of silver-halide film cameras. For imaging systems mounted on portable computers, cellular phones, etc. among those electronic cameras, size and weight reductions are now especially demanded. For an imaging system mounted on recently developed cellular phones, there is a growing demand toward an imaging optical system having performance high enough to take at least VGA class photographs (310,000 pixels).
For an optical system used with such imaging systems, there have been proposed numerous optical systems that are each composed of as few as two lenses yet that can have the VGA class capability by use of aspheric surfaces and achieve cost and weight reductions by use of plastic lenses.
Referring to an electronic image pickup device such as a CCD, on the other hand, as off-axis light beams leaving an imaging optical system are incident on an image plane at too large an angle, a microlens fails to perform its own light-condensation capability, offering a problem that the brightness of an image changes extremely between the central and the peripheral portion of the image. Thus, the angle of incidence of light rays on image pickup devices such as CCDs, that is, the position of an exit pupil is important in view of design. In other words, it is necessary to position an aperture stop at the front of a lens arrangement, so that the exit pupil position can be shifted as near to the object side of the lens arrangement as possible.
Some known imaging optical systems consisting of two aspheric lenses and using a plastic lens are disclosed in Patent Publications 1, 2, 3 and 4.
Several known imaging optical systems consisting of two aspheric lenses with an aperture stop located at the front of a lens arrangement are set forth in Patent Publications 5, 6, 7 and 8.
Patent Publications 9 and 10 show an imaging optical system consisting of two lenses with a first lens having weak refracting power. Patent Publication 11 proposes an imaging optical system of four-lens construction wherein some lenses are provided in the form of infrared cut filters.
Patent Publication 1
JP-A 2002-296496
Patent Publication 2
U.S. Pat. No. 3,393,186
Patent Publication 3
JP-A 2002-267928
Patent Publication 4
JP-A 2001-183578
Patent Publication 5
JP-A 8-334684
Patent Publication 6
JP-A 2003-75719
Patent Publication 7
U.S. Pat. No. 3,345,410
Patent Publication 8
U.S. Pat. No. 3,396,683
Patent Publication 9
JP-A 2002-296495
Patent Publication 10
JP-A 2000-35533
Patent Publication 11
JP-A 5-150172
However, these prior imaging optical systems are structurally difficult to achieve a sensible tradeoff between compactness and securing a back focus.